The present invention relates generally to automotive ignition systems, and more specifically to systems for minimizing ignition coil ringing effects resulting from coil activation.
Modern inductive-type automotive ignition systems commonly utilize power switching devices to control the flow of current through an ignition coil. Such devices are typically controlled so as to switch from an xe2x80x9coffxe2x80x9d state to a fully saturated xe2x80x9conxe2x80x9d state within a short time period, wherein such switching results in the voltage across the ignition coil changing rapidly from substantially zero volts to near battery voltage. The inductive nature of the ignition coil reflects and steps up this voltage across the primary coil to the secondary coil connected to an ignition plug, wherein the initial response of the secondary coil to this process may result in ringing. This ringing may, in some cases, create sufficient voltage across the spark gap of the ignition plug to cause a spark event. Such a mistimed spark event is undesirable and potentially damaging to the engine.
Referring to FIG. 1, one known example of an ignition system 10 of the type just described is illustrated in FIG. 1, wherein system 10 includes an ignition control circuit 12 having an electronic spark timing (EST) buffer circuit 14 receiving an EST control signal from a control circuit 16 via signal path 18. The EST buffer circuit 14 buffers the EST control signal and provides a buffered EST control signal ESTBF to a gate drive circuit 20. The gate drive circuit 20 is responsive to the ES TBF signal to supply a gate drive signal GD to a gate 22 of an insulated gate bipolar (IGBT) transistor 24 or other coil switching device via signal path 26. A collector 28 of IGBT 24 is connected to one end of a primary coil 30 forming part of an automotive ignition coil having an opposite end connected to battery voltage VBATT. An emitter 32 of IGBT 24 is connected to one end of a sense resistor RS having an opposite end connected to ground potential, and to a noninverting input of a comparator 36 via signal path 38. An inverting input of comparator 36 is connected to a reference voltage VR, and an output of comparator 36 supplies a trip voltage VTRIP to gate drive circuit 20. A secondary coil 40 is coupled to the primary coil 30 and is connected to an ignition plug 44 defining a spark gap 42 as is known in the art.
In the operation of system 10, gate drive circuit 20 is responsive to a rising edge of an ESTBF signal to supply a gate drive signal GD to the gate 26 of IGBT 24 as shown by the GD waveform 45 in FIG. 2A. As IGBT 24 rapidly begins to conduct in response to the gate drive signal GD, a coil current IC begins to flow through primary coil 30, as shown by the coil current (IC) waveform 47 in FIG. 2B, thereby establishing a xe2x80x9csense voltagexe2x80x9d VS across resistor RS. Due to the rapid turn on of IGBT 24 and subsequent rapid increase in voltage across the primary coil 30 to near battery voltage VBATT, ringing effects may result in the initial portion of the VSC waveform 49 as shown in FIG. 2C due to known LRC effects of the ignition coil. This ringing, as described hereinabove, may be sufficient to undesirably create a spark event across the gap 44 of ignition plug 42.
As the coil current IC increases due to the inductive nature of the ignition coil, the sense voltage VS across RS likewise increases until it reaches the comparator reference voltage VR. At this point, the comparator 36 switches state and the corresponding change in state of the trip voltage VTRIP causes the gate drive circuit 20 to turn off or deactivate the gate drive voltage GD so as to inhibit the flow of coil current IC through the primary coil 30 and coil current switching device 24. This interruption in the flow of coil current IC through primary coil 30 causes primary coil 30 to induce a current in the secondary coil 40, wherein the secondary coil 40 is responsive to this induced current to generate desired arc across the spark gap 42 of ignition plug 44.
To minimize, or at least reduce, ringing effects associated with the activation of coil current switching devices, a technique commonly referred to as xe2x80x9cphased turn-onxe2x80x9d, or PTO, has been developed. PTO reduces the ringing voltage illustrated in FIG. 2C by initiating the coil charging period with a carefully timed initial drive pulse to the coil current switching device (e.g., IGBT 24 of FIG. 1). Details relating to the foregoing PTO technique are described in U.S. Pat. No. 5,392,754 which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference. In accordance with the concepts described in the ""754 patent, the coil current switching device is initially turned on for a short time period (e.g., 2-7 microseconds), turned off for a similar time period, and then turned on again for the duration of the coil charging dwell period. The durations of the initial xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d periods are dependent upon the characteristics of the ignition coil being driven and are chosen such that the ringing created by the second turn-on is 180 degrees out of phase with the ringing produced by the initial turn-on. If the pulse timing is selected properly, this xe2x80x9cphasingxe2x80x9d of the coil response effectively damps the overall voltage response at the terminals of the secondary coil 40 and reduces the peak ringing voltage by as much as 50%. The use of this ring suppression technique can eliminate the need for an additional blocking diode in the coil assembly.
Another advancement in modern ignition systems is the use of multiple coil charging and spark events for a single combustion cycle. By generating multiple sparks in a rapid sequence, more spark energy can be delivered to the combustion cylinder than with a single spark event, thereby enhancing ignition of the air/fuel mixture. In accordance with this known technique, the coil current switching device (e.g., IGBT 24) is switched back on before all of the coil energy has been depleted, thereby recharging the primary coil 30 to its peak value from some intermediate coil current level as shown by the GD waveform 43 and IC waveform 46 in FIGS. 3A and 3B respectively. While only one recharging cycle is illustrated in FIGS. 3A-3C, it is known to use any desired number of recharge cycles, wherein systems of this type will be referred to hereinafter as multiple pulse ignition systems. One drawback to such a system, however, is that the unused energy within the ignition coil when the switching device is turned back on changes the coil""s response to the voltage transitions resulting from the switching of the coil current switching device. Referring to FIGS. 3A and 3C, for example, not only does the abrupt rising edge of the initial GD signal 43 result in a corresponding ringing of the secondary voltage VSC, as shown by waveform 48, but every rising edge thereafter of the GD signal 43 results in similar VSC ringing which may result in an unwanted spark event. The peak level of this ringing can be 50% higher than the baseline xe2x80x9cmakexe2x80x9d voltage (e.g., VBATT*coil turns ratio xe2x80x9cNxe2x80x9d), and may therefore cause mistimed spark events. Addition of a diode in series with the secondary coil 40 can also prevent a xe2x80x9cspark-on-makexe2x80x9d for a negative voltage system, although such a diode would interfere with potential ion current detection in an xe2x80x9cion sensexe2x80x9d ignition system, and a PTO technique is therefore critical for such applications.
What is therefore needed is an improved phased turn-on strategy. Such a modified PTO strategy should ideally be readily adaptable to any number of coil charging events to thereby minimize or at least reduce the resulting ringing events associated with the secondary coil voltage VSC in a multiple pulse ignition system. The strategy should further include provisions for adjusting the pulse widths of the PTO pulses to compensate for the known energy remaining in the coil as well as to account for variations in ignition system operating parameters such as engine speed, battery voltage and/or other engine operating parameters.
The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, an ignition control circuit comprises a first circuit producing first, second and third reference voltages, a capacitor, a second circuit responsive to a spark control signal to begin charging the capacitor, a third circuit responsive to a first mode control signal to enable an inhibit signal when a charge on the capacitor reaches the first reference voltage and to disable the inhibit signal when the charge on the capacitor reaches the second reference voltage, a fourth circuit responsive to a second mode control signal to enable the inhibit signal when the charge on the capacitor reaches the third reference voltage and to disable the inhibit signal when the charge on the capacitor reaches the second reference voltage, and a fifth circuit responsive to the inhibit signal to disable current flow through an ignition coil when the inhibit signal is enabled and to enable current flow through the ignition coil when the inhibit signal is disabled.
In accordance with another aspect of the present invention, a method of controlling an ignition system comprises the steps of charging a capacitor in response to a spark control signal, comparing a charge on the capacitor with a first reference voltage in response to a first mode signal, comparing the charge on the capacitor with a second reference voltage in response to a second mode signal, comparing a charge on the capacitor with a third reference voltage, the third reference voltage greater than the first and second reference voltages, disabling current flow through an ignition coil when the charge on the capacitor reaches either of the first and second reference voltages, and enabling current flow through the ignition coil when the charge on the capacitor reaches the third reference voltage.
One object of the present invention is to provide an ignition control circuit for implementing an improved phased turn-on ring damping strategy.
Another object of the present invention is to provide such a circuit hat is readily adaptable to a multiple pulse ignition system.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiments.